In many applications, it is desired to have information representative of a time elapsed between two events, be it an accurate or approximate measurement. An example of application relates to the time management of rights of access, especially to media.
The obtaining of this information representative of the elapsed time conventionally requires a time measurement by an electronic circuit powered, for example, by means of a battery, to avoid losing track of the information when the circuit is off.
It would be desirable to have a time measurement which operates even if the electronic measurement circuit is not powered.
An electronic device in which the time elapsed between two events is determined by measuring the charge of a capacitive storage element having an electrode connected to an electrode of a capacitive charge flow element leaking into its dielectric space has already been provided, for example, in PCT Application WO2008012459 (incorporated by reference). The storage element is charged when the device is powered, and its residual charge, after an interruption of the power supply, is measured when the device is supplied again. This residual charge is considered as representative of the time elapsed between the two device supply times.
The charge flow element comprises a region of smaller thickness in its dielectric space, capable of letting charges leak by tunnel effect. The storage element discharge speed is linked to the dimensions of the flow element leakage region. In particular, the storage element discharge speed increases as the thickness of the leakage region decreases and/or as the surface (seen from the top) of the leakage region increases.
A disadvantage is that, in practice, the sizing of the leakage region depends significantly on the considered technological manufacturing process. Indeed, the charge retention circuit is generally integrated on a chip comprising other components, for example, memories, logic blocks, etc. To avoid increasing the chip manufacturing cost, it is attempted to form the charge retention circuit with no additional manufacturing step with respect to the steps of manufacturing of the other components. In some recent technological processes, the dielectric layers available to form the leakage region of the flow element are too thin to enable the storage element to discharge slowly, even if the surface area of the leakage region is decreased to a minimum. As a result, the time measurement can only be possible, in the absence of a power supply, for a very short time, poorly adapted to most applications.